The present invention relates to a CCD (Charge Coupled Device) solid-state image pickup element with a MOS (Metal Oxide Semiconductor) structure which is used for an image input device such as a TV camera. Reference to FIGS. 1-4 will be made in the discussion of the prior art.
FIG. 1 is a block diagram of an interline transfer type CCD solid-state image pickup element. The element includes a vertical transfer clock 1, a horizontal transfer clock 2, an image signal output 3, a photosensor 4 (light receiving part) which forms an image, a vertical (CCD) shift register 5, a horizontal (CCD) shift register 6, and a readout pulse 7.
FIG. 2 is a diagram which shows the operating procedure of the image pickup element of FIG. 1. In FIG. 2, five photosensors 4 ar arranged vertically while four photosensors 4 are arranged horizontally forming a matrix type display screen. The vertical shift registers 5 are formed by five stages, while the horizontal shift registers 6 are formed by four stages.
Referring to FIG. 2(A), the image signals F.sub.ij (where i=1 to 5, j=1 to 4) are being accumulated in the photosensors 4 by the exposure and such image signals are ready to be read. Therefore, the vertical shift register 5 and the horizontal shift register 6 are in the idle condition.
When the readout pulse 7 is input to the photosensors 4, the image signals F.sub.ij, accumulated in the photosensors, are fetched by the vertical shift registers 5, one at a time, and thereafter the image signals F.sub.ji, which form the next image are accumulated in the photosensors 4. This condition is shown in FIG. 2(B).
Next, when the vertical transfer clock 1 inputs a pulse to the vertical shift registers 5, contents of the first stage F.sub.11, F.sub.12, F.sub.13, and F.sub.14 of the vertical shift registers 5 are transferred to the horizontal shift register 6. The condition after such a transfer is shown in FIG. 2(C).
Thereafter, when the horizontal transfer clock 2 inputs a number of pulses equal to the number of lateral picture elements (four, in this example) to the horizontal shift register 6, the contents of horizontal shift register 6 can be extracted in series at the image signal output 3. The condition after such an extraction is shown in FIG. 2(D).
Contents of the vertical shift registers 5 and horizontal shift registers 6 can all be read by repeating such readout operations by as many times as the number of vertical picture elements (five, in this example). Thereby, the readout of the image signals of one field, forming one frame, (in the non-interlace system) (one frame is formed by the image signals of two fields in the interlace system) by the raster scan system is completed.
FIG. 3(a) is a sectional view indicating the vertical sectional structure of the part M of FIG. 1, which includes the corresponding part of one photosensor 4 and one stage of the vertical shift register 5 also of FIG. 1 (in other words, the picture element structure of a MOS type CCD which provides the overflow drain structure for eliminating excessive signal charges which will result in blurring or smearing.
FIG. 3(b) indicates the energy level distribution in the above structure at time t.sub.1 and FIG. 3(c) indicates energy level distribution in the same structure at the time t.sub.2.
In FIG. 3, is included a picture element region 11, a channel stop 12, the input of one stage of the V-CCD 13 (corresponding to vertical shift register 5 of FIG. 1), a readout gate 14, a photosensitive part 15, a overflow control gate 16, an overflow drain 17, an optical shield of aluminum 18, a transparent electrode 19, insulation film 20, an electrode 21, a signal charge 22, and an overflow charge 23.
The signs "+" or "-" in the symbols are indications of P.sup.+, N.sup.+, and P.sup.- which indicates the doping concentration. The sign "+" means relatively high doping concentration, while "-" means relatively low doping concentration. P indicates a P type semiconductor, while N indicates an N type semiconductor. As shown, the bulk of the semiconductive substrate 40 in which the different regions are formed is P-type of relatively low doping.
FIG. 4 is a timing diagram of the vertical transfer clock 1 and a readout pulse 7 which is applied to the element structure indicated in FIG. 3(a).
The operations involving FIG. 3 and FIG. 4 are explained as follows. In FIG. 3(a), a bias voltage V.sub.SG is applied to the transparent electrode 19 by any means well known in the art. The vertical transfer clock 1 and readout pulse 7 are input to the electrode 21. When the vertical transfer clock 1 and readout pulse 7 are combined, the signal having three values (V.sub.H, V.sub.M, V.sub.L) as indicated in FIG. 4 is obtained.
The distribution of energy levels, where signal charges are accumulated, (for example, at time t.sub.1 of FIG. 4 in reference to FIG. 3(a) is indicated in FIG. 3(b). At time t.sub.1, when the readout pulse 7 is not applied, a bias voltage V.sub.SG is applied to the transparent electrode 19 and the energy level is lowered by as much as a certain level width V.sub.SG ' corresponding to the bias voltage V.sub.SG at the area between the photosensitive part 15 and the overflow drain 17. As a result, a well for accumulating charges is formed.
When the vertical transfer clock 1 is applied to the electrode 21, the energy level is lowered by as much as the level width V.sub.M ' or V.sub.L ' which corresponds to voltage levels V.sub.M or V.sub.L of the transfer clock 1, at the area between the overflow drain 17 and the readout gate 14. As a result, a well for accumulating charges is formed.
Here the following relation exists as is apparent from FIG. 4. EQU V.sub.M '&gt;V.sub.L '
In FIG. 3(b), when a voltage barrier generated at the junction area between the photosensitive part 15 and the overflow control gate 16 is assumed as P.sub.B, while V.sub.SG '-V.sub.M '=P.sub.A, then the relation P.sub.A &gt;P.sub.B can be obtained.
When the light is incident on the solid-state image pickup element, electrons are excited at the areas not covered with the optical shield 18, which is made of aluminum, and said electrons are accumulated in the charge well of the photosensitive part 15.
The amount of charges accumulated in this well is limited by the voltage barrier P.sub.B. The charges 23, overflowing said barrier P.sub.B, flow to the overflow drain 17, passing the overflow control gate 16, and are finally discharged to the outside of the image pickup element.
Usually the voltage barrier P.sub.B is determined so that the amount of charges on the photosensitive part 15 does not exceed the amount of charges to be handled by V-CCD 13. Its size is determined by the difference between the impurity concentrations of the semiconductor which forms the photosensitive part 15 and the semiconductor which forms the overflow control gate 16.
The effect of time t.sub.2, where the readout pulse is applied, is explained with reference to FIG. 3(c). When the readout pulse 7 is applied to the electrode 21, the level is lowered as much as the level width V.sub.H ' in accordance with the voltage V.sub.H of said pulse 7 at the area between the overflow drain 17 and readout gate 14 (when V.sub.H '-V.sub.M '=P.sub.c, the energy level is further lowered to as much as the voltage barrier P.sub.c in comparison with FIG. 3(b)).
In this case, the following condition is necessary. EQU V.sub.H '.gtoreq.V.sub.SG '
Thereby, the energy level of readout gate 14 is lower than that of the photosensitive part 15 and the signal charges 22 flow into V-CCD 13 passing through the readout gate 14 for the read operation. Here, a channel stop 12 is provided between V-CCD 13 and the overflow drain 17 of the adjacent picture element therefore preventing the charges 22 on V-CCD 13 from leaking out.
At time t.sub.3 where the vertical transfer clock 1 is applied, when V.sub.SG '-V.sub.L '=P.sub.A ', since P.sub.A '&gt;P.sub.A, the difference of the energy level between the readout gate 14 and the photosensitive part 15 becomes larger.
A CCD solid-state image pickup element of the prior art as explained previously requires the time of 1/60th of a second to read the image signals of as much as one field from the pickup element. Moreover, in the case where the image signal employs the non-interlace system, one display screen corresponds to one field (1/60 second), but in the case where the interlace system is employed, one display screen corresponds to two fields (1/30 second), and the signal charges are accumulated at the photosensitive part 15 for a period of one display screen (therefore, the period of such accumulation corresponds to the shutter speed in the case of a camera which uses a film).
The period for accumulating the signal charges continues comparatively as long as 1/60th of a second but it is not as long as the shutter speed. Therefore, the problem of blurring occur when the object is moving.